Long-Term Heat Storage Device and Method for Long-Term Heat Storage of Solar Energy and Other Types of Energy with Changing Availability

ABSTRACT

The invention relates to a long-term heat storage device for long-term storage of solar energy and other types of energy, in the heat storage material of which a rock bulk material, in particular of volcanic origin, such as diabase, basalt, granite and gneiss, is used. The rock bulk material forms a polydisperse bulk material, in particular as the void volume of the rock bulk material (granulate) having a first particle size or particle size distribution takes up a granulate having a second particle size or particle size distribution. The rock bulk material can be enclosed by a bulk powder fill, in particular an ash fill, in particular with a shell of shaped rocks interposed. The rock bulk material can be enclosed, all around or predominantly, at least laterally, by a shell of shaped rocks which is in particular cylindrical, in particular by a masonry wall.

FIELD OF THE INVENTION

The invention relates to a long-term heat storage device for long-termheat storage of solar energy and other types of energy with changingavailability. The long-term heat storage device comprises a heat-stagemass having an insulating layer surrounding said mass according to thepreamble of claim 1. The invention further relates to a method for thelong-term heat storage of solar energy and other types of energy withchanging availability by means of a long-term heat storage deviceaccording to the preamble of claim 27. The aim of the invention is a newtechnical concept to ensure continuity when using solar energy and othertypes of energy with changing availability throughout the entire year.The concept essentially assumes that the solar radiation or the othertype of energy with changing availability is highly concentrated withthe result that high temperatures are achieved, with the result that theenergy of the heat storage mass is increased and the energy efficiencyof the system is raised. The energy collected from the sun or fromanother energy source with changing availability is stored in along-term heat storage device with low heat losses during the wintermonths. The new concepts assume an efficient transfer of the solarenergy or the other type of energy with changing availability to a heattransfer medium. The thermal energy is then transported into thelong-term heat storage device and to the heat storage mass.

The insulation and the geometry of the heat storage device are also partof the new concept. The new type of insulation should ensure a long-termstorage of the thermal energy in a particularly favourable manner wherethe heat losses are kept low.

TECHNOLOGICAL BACKGROUND

The major problems in the world's energy supply are becomingincreasingly obvious, in particular due to the incidents which aretaking place increasingly frequently in existing nuclear power plants.It is becoming increasingly clear that the safety of the world is beingincreasingly endangered by nuclear energy.

Already a large number of new solar, tidal and wind power plants havebeen built in Europe and the USA but these do not by any means meet therequirements imposed, on account of the changing availability of therespective energy source. In any case, there are only a few places onthe Earth where, for example, the sun shines 365 days×8 hours a day. Thepresent prior art does not allow power to be produced from solar energyon sunny days for 24 hours a day without needing to use energy from anadditional energy source in this case, and the same applies accordinglyfor other types of energy with changing availability, such as waterpower and wind power. Recently, thermal storage devices are being usedin solar thermal power plants so that these can also be operated incloudy conditions or after sunset. In order to bridge sunless days withenergy, various long-term heat storage devices are presently beingdeveloped such as: salt storage devices, concrete storage devices,compressed air storage devices, sand storage devices and pump storagedevices.

Salt storage device: The excess heat is stored in a liquid salt mixtureof 60% sodium nitrate (NaNO₃) and 40% potassium nitrate (KMO₃). Bothsubstances are used inter alia as fertilizers and for preservation infood production. The liquid salt storage devices operate at atmosphericpressure and consist per power plant of two tanks for example having aheight of 14 m and a diameter of 36 m. When pumping from the “cold” intothe “hot” tank, the liquid mixture at an initial temperature of 290° C.Celsius absorbs additional heat so that it is heated to about 390° C.Celsius. A full storage device can operate a turbine for 7.5 hours. Thestorage capacity of this salt storage device varies around about 80-100kWh/m³. The disadvantage is that the temperature of the salt mass mustnot fall below 290° C. so that additional electrical heating must beprovided so that the salt mass does not solidify.

Concrete Storage Device:

The German Aerospace Centre (DLR) together with industrial partner Ed.Zublin AG has presented a new thermal storage device for solar powerplants. The pilot plant is based on the storage of heat in concrete attemperatures up to 400° C. The heat transfer medium is thermal oil whichis transported through steel pipes which are embedded and cast into theconcrete mass. The moulds of concrete are of modular structure andspecifically such that the entire storage device consists of individualmodules and can be used for any powers. The production costs arerelatively high relative to the specific storage capacity. In order toachieve a high specific storage capacity, the storage temperature mustbe substantially higher. For cost reasons these storage devices wouldnot be suitable as long-term storage devices.

Compressed Air Storage Device:

For wind power plants heat reservoirs play only a subsidiary role. Windplants already deliver usable power. In order that excess kilowatt hoursin strong wind phases are not lost, they should drive compressed airpumps. These press compressed air into subterranean cavities such as,for example salt caverns. If the power requirement increases, the aircan flow out again and set the turbine wheels of generators in rotation.However the compression and escape of compressed air is not trivial.This is because during compression the air is heated to above 600° C.Conversely it cools down when flowing out. A high efficiency of thecompressed air storage device of about 70 percent can only be achievedif the compression heat is also stored and subsequently heats theoutflowing air again. Major costs are involved in implementing thisrequirement. From this it follows that a heat storage device must beadditionally provided inside a compressed air storage device in order tobe able to store the thermal energy separately so that the highefficiency can be maintained. As a result, the entire process isburdened with additional costs. It is no easy task to achieve thenecessary air tightness at 100 bar in underground cavities. This type ofenergy storage device is therefore not in a position to store energyover longer time intervals.

Sand Storage Device:

A sand storage concept has been developed at the Solar Institute Jülichof the FH Aachen, in which quartz sand is used as storage medium. Thisproject was supported by the Federal Ministry for the Environment,Nature Conservation and Reactor Safety, by the Ministry of EconomicAffairs and Energy of the State of North-Rhine Westphalia, by the GermanAerospace Centre e.V, by Aachen University of Applied Sciences, by theBavarian State Ministry of Economic Affairs, Infrastructure, Transportand Technology, by Munich power plants and by the Solar InstituteJülich. These research projects have been adequately reported inwww.fz-jülich.de and in www.taz.de and in www.wdr.de. This conceptpromises a reduction in storage costs if an efficient transfer of air tosand can be achieved. The aim of this project is the investigation anddevelopment of a suitable air-sand heat transfer medium prototype forthis purpose, which operates in the temperature range of 400-900° C. Inthe project a corresponding experimental structure was designed,manufactured and measured. The principle: so-called heliostatsconcentrate solar radiation onto a receiver at the top of a solar tower.Here the porous structure in the receiver is heated, its heat is in turndelivered to air which is sucked in from the surroundings. The airheated to max. 900° C. is supplied to the air-sand heat transfer mediumfor heating the sand. The heated sand is passed via a simple drop tubeinto a hot storage device and from there as required further into thefluidized bed cooler. The fluidized bed cooler, an aggregate which isused in a conventional fluidized bed incineration, drives a steam cycle.The generated steam is finally fed into a conventional power plantprocess of a steam turbine which drives the generator to produce power.The cooled sand leaves the fluidized bed cooler at a temperature ofabout 150° C. and is either fed back to the air-sand heat transfermedium or is stored in a cold storage device. The sand storage devicehas been developed by the Solar Institute Jülich with the aim of beingbuilt in the Sahara solar power plant where sand is available inabundance and is free. The assessment of the new sand storage system canbe summarized as follows: the heat transfer in moving sand having asmall grain diameter is high and this is the advantage. However the riskof abrasion in the sand is high. The power consumption in the sandfluidized bed is high, the specific storage capacity of the sand is highand the entire stored quantity of heat cannot be large because thequantity of sand is relatively small for process technology reasons.This heat storage device cannot be considered to be a long-term heatstorage device.

Pump Storage Device:

A pump storage power plant (also called pump storage plant PSW) is aparticular form of a storage power plant and is used to store electricalenergy by pumping water uphill in phases of energy excess. This watercan subsequently be allowed to flow downhill and thereby produceselectrical power again by means of turbines and generators. Theelectrical energy is therefore stored by converting into potentialenergy of water and is fed into the mains by converting this potentialenergy back into electrical energy. The technical development of pumpstorage power plants in the last few years makes it possible today tooperate a hydroelectric power plant as a pump storage power plant withan overall efficiency of more than 75% for green power production. Thepump storage device is today provided as a component of a solar or awind plant. The disadvantage is that the natural conditions must besuitable for the use of a pump storage device. Enormous amounts of waterare required for relatively low electrical powers and therefore such astorage system is very cost-intensive. Enormous basin areas, largeheight differences, pipelines having a diameter of several metres and avery expensive infrastructure are required. The storage device can onlybe provided at specially suitable sites. This type of storage device isin any case not suitable for storing energy over several months.

In summary the present prior art can be briefly described such that thetechnology level relating to ensuring continuity when using solar energyis far from satisfactory. At present it is not yet possible to producepower 365 days×24 h throughout the entire year from solar energy. Aslong as this aim is not achieved, solar energy or other types of energywith changing availability cannot be regarded as a reliable source ofenergy. This aim should be implemented according to the invention.

The complete replacement of conventional types of energy such as fossilor nuclear fuels by alternative energies is only possible through thedevelopment of appropriate energy storage systems. The sustainableenergy is stored and held in readiness over fairly long periods of time.The energy buffer storage device which is capable of compensating forcertain availability fluctuations in order to ensure continuity in theenergy supply is required today. Of all the sustainable energies, windenergy is the most commonly encountered today, in particular in WesternEurope and in the USA. Some countries have today provided a fraction ofabout 5% of the total energy production through wind energy. That is anappreciable amount of energy. However, the typical availabilityfluctuations can cause major problems in the energy supply. In thiscase, coal or gas power plants or nuclear power plants must be operatedfor a short time. This is technically very demanding and expensive.However nuclear power plants are usually only suitable for base loadoperation. However appreciable problems also arise when there is anexcess of wind energy. The energy must be briefly removed to avoid thecollapse in the energy supply.

The storage systems presently being developed therefore do not meet therequirements imposed: the salt storage device has a low storage capacity(80-100 kWh/m³) It operates in the range of relatively low temperatures(about 400° C.). In addition, the temperature of the salt mass must notfall below 290° C. therefore additional electrical heating must beprovided so that the salt mass does not solidify. A concrete storagedevice also has a low storage capacity and is expensive to manufacture.It is operated with a thermal oil and the operating temperature is ofthe order of magnitude of 400° C. At this temperature a large specificstorage capacity cannot be expected. A considerable quantity of steelpipes is incorporated in the concrete storage device. Steel and concretehave different expansion coefficients and there is always the risk thatcracks will form in the concrete storage device which prevent heatconduction and cause an insulating effect. The sand storage devicecertainly operates at high temperatures and the heat transfer is highbut the risk of abrasion is very high. The power consumption fortransporting the sand is accordingly very high. The use of sand ispossibly only appropriate in the Sahara—less so in Europe. The storedquantity of heat cannot be large because the amount of sand isrelatively small for process technology reasons. This heat storagedevice cannot be considered to be a long-term heat storage device. Forthe compressed air storage device the situation appears better.Certainly a large amount of energy can be stored in the undergroundstorage device. However other process technology grounds are presentwhich cannot be easily overcome. During compression of air, thetemperature rises. If the compressed air is stored in the undergroundstorage device, cooling of the air is inevitable and as a resultappreciable energy losses occur. On the other hand, it is technicallydemanding to get the underground cavern airtight. To provide anintermediate storage device after the air compression in order to storethe thermal energy would incur additional costs. The use of the pumpstorage device requires suitable natural conditions. The completeinfrastructure for a pump storage device is considerably more expensive.Enormous basin areas are required to hold the large amount of water inreadiness. This solution cannot be generalized and is rarely applicable.Enormous amounts of water are required for relatively low electricalpowers and therefore such a storage system is very cost-intensive. Thistype of storage system is in no way suitable for storing energy overseveral months.

Known from DE 10 2010 008 059 A1 is a long-term heat storage devicewhich comprises a solid body having a high heat storage coefficientwhich is suitable for high temperatures and is thermally loaded. Thetype of solid body is not described; however it should be possible touse a quartz sand bulk material as is known for heat storage. In anycase, its thermal loading should be accomplished by heat conduction. Theheat conducting device provided for this which should have a highthermal conductivity and a high temperature resistance is also notdescribed in detail here. A heat absorption unit for converting thesolar light is located upstream of the heat conducting device and notdescribed in detail. The entire combination of heat absorption unit,heating conducting device and heat storage device should overall bethermally insulated by an ultra-insulation. It remains unclear how thisshould be achieved. An energy converter unit is located in the heatstorage device to decouple the energy from the heat storage device. Theworking temperature of the long-term heat storage device should bebetween 100 and 800° C. Year-round heating of apartment and officeblocks should be possible.

It can be seen from this that none of the concepts put forward so farfor the long-term storage of energy meet the requirements or have provento be capable of implementation in practice.

DESCRIPTION OF THE INVENTION

It is the object of the invention to provide an inexpensive system forutilizing solar energy and other types of energy with changingavailability, according to which the solar energy or other type ofenergy with changing availability which has been concentrated up to amaximum temperature, e.g. by means of a concentrator and transferred toa heat transfer medium as heat in an absorber, is delivered by means ofthe heat transfer medium to the heat storage mass which is located inthe long-term heat storage device. The heat storage mass should becapable of being heated to a temperature of about 1000° C. The long-termheat storage device should be insulated in such a manner that thethermal energy can be held in readiness over a long period of time (afew months up to half a year) with low thermal losses. To solve thisobject a generic long-term heat storage device having the features ofclaim 1 and a generic method for long-term thermal storage of solarenergy and other types of energy with changing availability having thefeatures of claim 27 is proposed. Accordingly the invention proposes ageneric long-term heat storage device comprising a thermally insulatedsolid bed of a rock bulk material serving as heat storage mass,comprising a material suitable for high operating temperatures, inparticular for temperatures between 100 degrees C. and 1000 degrees C.,preferably between 300 degrees C. and 900 degrees C., having a highspecific heat capacity, in particular greater than 600 J/kgK, highthermal conductivity, in particular higher than W/mK and high density,in particular greater than 2 kg/dm³, preferably of volcanic origin suchas diabase, basalt, granite and/or gneiss, according to claim 1 andfurther a generic method for the long-term heat storage of solar energyand other types of energy with changing availability according to claim27, whereby a granular material (rock bulk material), in particularhaving a high specific heat capacity, high thermal conductivity and highdensity, in particular of volcanic origin such as basalt rock, diabaserock, granite rock and/or gneiss rock, is provided in a solid bed asheat storage mass. The granular material is insulated in a suitablemanner so that the heat losses remain low over a long period of time.The heat insulating material preferably has a plurality of contactresistances and/or a low thermal conductivity and/or a spongy, fibrousand/or porous structure and particularly preferably comprises amicronized powder such as micronized ash. The thermal insulation is ofindependently inventive importance. The granular material of the heatstorage mass is heated to a high temperature by means of a heat transfermedium interacting with the granular material, in particular the heattransfer fluid flowing through the granular material, and is insulatedin a suitable manner so that the heat losses remain low over a longperiod of time.

It is now possible to implement the invention in various ways. It hasbeen shown inter alia that the geometry of the long-term heat storagedevice has a substantial influence on the heat losses. The geometry ofthe heat-stage device should be optimized in this sense. Furthermore anoptimal insulation and an optimal thickness of the insulating layer havebeen found. It has further been shown that the relative heat losses ofthe heat storage device decrease with increasing storage capacity. Ithas further been shown that a rock bulk material of volcanic origin issuitable for high operating temperatures and at the same time has goodphysical properties such as high specific heat capacity, high thermalconductivity and high density. Preferably types of rock of volcanicorigin come into consideration here such as basalt rock, diabase rock,granite rock and gneiss rock. In order to be able to achieve anefficient insulation of the heat storage mass over long periods of time,a heat insulating material with good insulating properties is used. Thisheat insulating material should inter alia be inexpensive in order tokeep the economic viability of the plant sustainable. The heatinsulating material should have a plurality of contact resistances.Furthermore, convective flow inside the insulating layer should beprevented. As a result of the large number of contact resistances, theheat transfer resistance due to radiation is increased. The moreparticles lie adjacent to one another, the greater is the number of heatconduction resistances. As a result of the large number of contactresistances, the heat conduction resistance due to convection and due toradiation is comparatively high. The heat conduction between twoparticles takes place at a point through contact. If the distancebetween two particles at the contact point is shorter than the freemolecular path length, the thermal conductivity of the air decreases bya factor of 10 and acts as insulation. The smaller are the particles andaccordingly the larger the number of contacts, the higher is the heattransfer resistance. It was found that these properties could be foundin micronized powder. A micronized powder in the sense of the inventionis understood as a powder in which the particle diameter is less than100μ (micron). The powder can be obtained, for example, by fine grindingor from the electric filter in power plants. Micronized ash fromelectric filters in coal power plants can be considered to beparticularly suitable. Among the various types of micronized powders, inparticular types of ash, those having a low bulk density (in particularof 500 kg/m³ to 1000 kg/m³) were found to be particularly suitable. Athermal conductivity of the micronized power layer of about λ=0.09 W/mKto about λ=0.01 W/mK, in particular of about 0.03 W/mK can be seen asthe criterion for a good insulating property. Accordingly, the thermalconductivity of the insulating material should be low, which is why theinsulating material should preferably have a spongy or porous structure.It was further found that not only the insulating material is importantfor good insulation but also the geometry of the heat storage mass andthe heat storage device with the insulation acquires a particularimportance. An optimized insulating layer thickness is also important.It is also important that heat bridges are maximally avoided.

A long-term heat storage device according to the invention preferablysatisfies the following conditions inter alia: the heat storage mass isresistant to high temperatures of several hundred to thousand degreesCelsius; the heat storage mass is present in the form of granularmaterial in order to have sufficient heat exchange area for heattransfer; the granular material is resistant to temperature change; thegrain size of the granular material lies in a certain order of magnitudein order to keep the pressure losses of flowing air or another heattransfer fluid through the bulk material within the framework; the heatinsulation of the heat storage mass has a relatively low thermalconductivity and is inexpensive and available in large quantities; theinsulation thickness and the geometry of the heat storage device arematched to one another so that the heat losses of the bulk material arereduced to a technical or economically meaningful minimum; the capacity,the size and the geometry of the long-term heat storage device arematched with the storage time so that the heat losses for theserelationships are minimal.

The grain size of the rock bulk material of the heat storage mass can bedesigned according to another aspect of the invention as a polydispersesystem so that the desired bulk material density is achieved andtherefore the specific heat storage capacity can be brought to a desiredvalue. A polydisperse system in the sense of the invention is understoodas a multigrain bulk material. The multigrain bulk material can becomposed so that the porosity or the intermediate grain volume of thebulk material acquires a certain value. A single-grain bulk material(that is, for example a ball bulk material consisting of balls of thesame diameter) is designated as a monodisperse system on the other hand.The bulk material density of a multigrain bulk material and thereforealso the specific heat storage capacity of the bulk material can besubstantially enlarged by skilful mixing ratios of the grain sizespectrum. This is preferably accomplished whereby the void volume of thegranular material having a first grain size or grain size distributionis filled by a granular material having a second grain size or grainsize distribution. Here, bearing in mind the band widths of the fine andcoarse bulk material, as is particularly preferred it is possible totalk of a quasi bidisperse bulk material. Both granular matters can beof the same or different species/type.

Preferably a rock bulk material of volcanic origin is used as the heatstorage mass. Types of rock which are particularly preferably used asheat storage mass are:

True Specific Thermal Elastic density heat conductivity modulusElongation Rock kg/dm³ J/kgK W/mK N/mm² 1/K Basalt 2.85-3.05 800 1-1.69-10 × 10⁴ 1-7 × 10⁻⁸ Diabase 2.75-2.95 810-900 2-4  9-10 × 10⁴ 2-5 ×10⁻⁸ Granite 2.6-2.8 790 2.8  4-8 × 10⁴ 3-8 × 10⁻⁸ Gneiss 2.65-2.851000  2.9 4-10 × 10⁴ 2-7 × 10⁻⁸

The heat storage mass should be capable of being heated to at least 800°C. Assuming a mean void volume of the rock bulk material of 40% (ε=0.4),this gives a mean bulk density of about 1740 kg/m³. In this case, thetrue density of the diabase rock ρw=2900 kg/m³ is taken as the basis. Ifthe rock bulk material is configured as a polydisperse system such as,for example, as a bidisperse system, as mentioned above, the bulkmaterial density and therefore also the specific heat storage capacityof the rock bulk material can be substantially enlarged by skilledmixing ratios of the grain size spectrum, as illustrated in theexemplary embodiments.

It has been found that the usable storage capacity depends substantiallyon the method or on the type of use of the stored energy. For example,when the storage device is used for the purpose of heating buildings,the temperature in the storage device can be run down from, e.g. 800° C.to 100° C. Here Δt=700° C. In power production the cooling of the heatstorage device is run down from e.g. 800° C. to 400° C. and in this caseΔt=400° C. When using the heat for heating, the storage capacity is thentherefore greater. The use of stored solar energy for heating or coolingbuildings has the advantage that for heating the heat storage mass canbe cooled down to 100° C. If on the other hand power is produced bymeans of an air machine which operates according to the Carnot cycle, itis possible to work with a high efficiency of 70% (at a temperature ofthe working medium of 700° C.) down to a diminishing efficiency of 21%(at a temperature of the working medium of 100° C.). It is difficult toenvisage that a steam turbine can operate in the variable pressure rangeof, for example, 200 bar at 500° C. to 10 bar at 180° C. If on the otherhand an air machine is heated externally by means of the energy storedaccording to the invention and depending on how the temperature of theheat source decreases, the air pressure in the machine is reducedaccordingly, e.g. automatically, the maximum possible Carnot efficiencyfor the respective temperatures of the heat source is thereby extracted;this is naturally taking into account heat and friction losses.

The working medium in the air machine moves in a closed cycle wherebygood conditions are created for achieve a high efficiency during theenergy decoupling from the heat storage device.

In a practical embodiment of the invention, it is a question of suitablytaking into account the computational, theoretical andprocess-technology relationships in order to achieve an optimum for thelong-term heat storage with the lowest heat losses. As alreadymentioned, the geometry of the heat storage device also plays animportant role. It can be demonstrated that the heat storage losses in along-term heat storage device having a spherical geometry are thelowest, after this comes a cylinder geometry in which the height isequal to the diameter, cubes and parallelepipeds in which the heatlosses are highest.

One embodiment according to the invention provides that the heat storagemass preferably has a cylinder shape (diameter=height of the cylinder).In order to achieve this, the rock bulk material is inserted into acylindrical brick (annular) masonry wall. A circular plate, preferablyof steel, can be placed on a foundation, preferably of concrete. Anannular masonry wall, preferably made of solid brick and by means ofmortar which is suitable for high temperatures (800° C.) is erected onthe plate in a circular shape. A micronized powder, such as micronizedash which accumulates, for example, in an electric filter, can be usedas ground insulation for the heat storage mass. The layer thickness ofthe micronized powder is determined so that the heat losses are minimal.

The in particular micronized powder layer can preferably be surfaced intwo layers, with stone, in particular of brick. A small-grained finebulk material of the same origin as the heat storage mass is, forexample, scattered and levelled on the stone layer or layers. A finebulk material in the sense of the invention is understood as a bulkmaterial which, unlike the filling and coarse bulk material of the heatstorage mass, has a smaller mean grain diameter. For example, as ispreferred in this respect, the bulk material fill can have a grain sizebetween 30 and 60 mm and the fine bulk material can have a grain sizebetween 10 and 20 mm. On and/or between the fine-grained bulk material(fine bulk material), for example, made of chamotte brick, supplychannels for a heat transfer fluid, in particular air, are formed sothat air can flow through these as heat transfer medium and be uniformlydistributed over the ground of rock bulk material. The aim is todistribute the heat transfer fluid uniformly over the rock bulkmaterial. In order to achieve this, the supply channels are distributeduniformly over the contact surface. The height of the rock bulk materialis ideally equal to its diameter. The annular masonry wall encloses therock bulk material over the entire height and at the tip of the rockbulk material, discharge channels for the heat transfer fluid are formedin the same way as the supply, in particular made of chamotte brick.Likewise the intermediate spaces between the supply and/or dischargechannels are filled with rock bulk material. The air flowing through therock bulk material is removed outwards through the discharge channelsand if desired is held in circulating motion. The rock bulk materialends at channel height. Furthermore, preferably the fine-grained bulkmaterial, preferably of the same origin as the correspondingly coarserrock bulk material is applied on which the air channels of chamottebricks are laid. The fine-grained bulk material can improve the layingof the bricks. As result of the relatively larger flow resistance, abetter and more uniform air distribution can be achieved due to the finebulk material. The fine-grained bulk material can again be surfaced, inparticular in two layers, with conventional stone, in particular ofbrick. The heat storage mass is thereby enclosed on all sides. At acertain distance from the brick masonry wall, a cylinder, preferablymade of steel, can be located, which encloses the entire area. Theintermediate space between one such jacket and the annular masonry wallcan be filled with micronized powder such as micronized ash which servesas insulation for the heat storage mass. Preferably a uniform insulationthickness is provided around the brick masonry wall in order to keep theheat losses to a minimum. Here the finding is very important that eachenergy content of the heat storage mass can be assigned an optimal layerthickness of thermal insulation in which the heat losses are minimal.With increasing layer thickness of the insulating layer, the diameter ofthe heat storage device increases accordingly and therefore the outerheat transfer area. Here also the economic viability of the whole is animportant factor so that sometimes higher heat losses can be acceptedand specifically as a compromise between the heat losses and theinvestment costs. Naturally the costs of the stored energy play animportant role. On the other hand: in the case of greater heat lossesthe concentrator possibly used must accordingly be designed to be largerto cover these losses. It was found that the optimal solution should besought by optimizing the heat losses.

The relationships can best be illustrated by means of an example: Weassume that a house having a living area of 150 m² and a room heightH=2.6 m is to be heated. 390 m³ is to be heated for which an averageheating power during the 6 months (from November to April of the nextyear) of about 12 kW is required, or the total amount of energy during 6months is 30600 kWh.

A storage device can be designed with the diameter of the heat storagemass Dsm=4.5 m and with the height of the heat storage mass Hsm=4.5 m.Diabase rock of volcanic origin having a bulk density of 2100 kg/m³ isassumed for the heat storage mass.

The grain diameter of the rock bulk material lies between 30-60 mm andthe bulk material consists of the 15-30 mm fraction in order to achievea bulk density of 2100 kd/m³. The granular material is inserted in acylindrical (annular) masonry wall of 250 mm wall thickness. The brickmasonry wall is filled with a 1.4 m ash layer and over this in twolayers, 120 mm of brick as surfacing and above the brick, a fine-grainedbulk material 100 mm thick is uniformly distributed. A cladding of sheetmetal 3 mm thick is provided around the brick masonry wall. Between thebrick masonry wall and cladding, the ash layer 1.4 m thick is providedas insulating layer. Above the rock bulk material the ash layer 1.4 mthick is also provided for insulation. The external dimensions of theheat storage device are therefore: diameter about 8 m and height about 8m.

With these dimensions and the insulation concept shown, the heat lossesfor the period from April to November during the storage of solar energycan be calculated as 7624 kWh which is about 21% in relation to thestorage capacity of 36700 kWh.

It is shown that for greater storage capacity relative losses becomeincreasingly smaller. If we make a design for 10000 m² living area (200dwellings), it will be shown that the relative losses are in the orderof magnitude of 4% and for a living area of 100000 m² (2000 dwellings)relative losses are in the order of magnitude of about 1.6%.

From this it can be concluded that the concept of the long-term heatstorage device according to the invention offers an exceptional scopefor storing solar energy in the summer months and providing full heatingduring the winter months.

The aforesaid components and those claimed and described in theexemplary embodiments to be used according to the invention are notsubject to any particular framework conditions in their size, shape,material selection and technical conception so that the selectioncriteria known in the field of application can be used unrestrictedly.

Further details, features and advantages of the subject matter of theinvention are obtained from the subclaims and from the followingdescription and the relevant drawings in which, as an example, anexemplary embodiment of a long-term heat storage device is shown.Individual features of the claims or the embodiments can also becombined with other features of other claims and embodiments.

BRIEF DESCRIPTION OF THE FIGURES

In the drawings:

FIG. 1 shows in vertical section a schematic diagram of the long-termheat storage device filled with rock granular material of volcanicorigin which is inserted in a closed cylindrical brick masonry wall. Airdistribution channels are located at the bottom of the rock granularmaterial and air collecting channels are provided at its top; aninsulating ash layer is provided around the enclosing brick masonry wallin order to keep the heat losses to the outside low or prevent them;

FIG. 1A shows a cross-section along the line A-A according to FIG. 1through the long-term heat storage device. The air distribution channelsfor heating air can be seen at the bottom of the rock bulk material;

FIG. 1B shows in plan view the air distribution channels according toFIG. 1 and FIG. 1A made of chamotte brick;

FIG. 1C shows a detail FIG. 1 for the arrangement of the chamotte bricksto form air channels;

FIG. 1D shows in detail a cross-section through the main distributionchannel;

FIG. 1E shows an alternative possible embodiment to FIG. 1 whereby theentire brick masonry wall lies on the insulating ash layer. As a resultof this type of design, the heat losses of the heat storage device aresmall but this embodiment is more difficult from the productiontechnology viewpoint;

FIG. 1F shows a detail “A” from FIG. 1, namely: the rock bulk material,the insulating ash layer, the brick masonry wall, the air distributionchannels, the surfacing of the ash layer by means of the bricks and thefine-grained bulk material;

FIG. 2 shows the long-term heat storage device according to FIG. 1 withexternal cladding;

FIG. 2A shows across-section through the long-term heat storage devicewith external cladding along the line A-A according to FIG. 2;

FIG. 3 shows a possible embodiment of the external cladding of thelong-term heat, storage device made of sheet metal shells which aremounted in segments;

FIG. 4 shows a plan view of the cladding according to FIG. 3;

FIG. 5 shows an improved embodiment of the long-term heat storage devicewith an additional insulation which is applied over the externalcladding and a protective sheet for the insulation;

FIG. 5A shows a cross-section through the heat storage device accordingto FIG. 5;

FIG. 6 shows a possible embodiment for the long-term heat storage devicewhich is disposed in the ground;

FIG. 6A shows a cross-section of the long-term heat storage deviceaccording to FIG. 6;

FIG. 7A shows a diagram of the heat losses of a heat storage device(cylindrical shape with D=H) as a function of the insulation thicknessof the ash which stores the solar energy from April to November; thestorage capacity corresponds to a living area of 150 m² (3 apartments orone house);

FIG. 7B shows the heat losses and (as an example) the efficiency of aheat storage device (cylindrical shape with D=H) as a function of theinsulation thickness of the ash which stores the solar energy from Aprilto November; the storage capacity corresponds to a living area of 10,000m² (200 apartments);

FIG. 7C shows the heat losses of a heat storage device (cylindricalshape with D=H) as a function of the insulation thickness of the ashwhich stores the solar energy from April to November; the storagecapacity corresponds to a living area of 100,000 m² (2000 apartments);

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

According to FIG. 1, a rock bulk material 1 of volcanic origin isinserted in a cylindrically walled (masonry wall 2) cavity. The masonrywall 2 cylindrically surrounding the cavity lies on a plate 3, e.g. madeof steel which in turn lies on a foundation 4 made of concrete. Theplate 3 serves, inter alia, to prevent possible diffusion of moisturefrom the foundation 4 into the ash layer (bulk powder fill 5) locatedthereover. The bulk powder fill 5 (hereinafter also designated as ashfill 5) is used for base insulation of the rock bulk material 1. The ashfill 5 is surfaced at its top in two layers by means of rock 6preferably consisting of brick (hereinafter also designated as bricks 6)in order, inter alia to prevent ash dust from entering into the streamof the heat transfer agent preferably consisting of air. A fine-grainedbulk material 7 is placed on the bricks 6 on which air supply channels,in particular in the form of an air supply channel 8 (hereinafter alsodesignated as main air supply channel 8) and air supply distributionchannels 8 a branching therefrom are laid. The fine-grained bulkmaterial 7 is preferably provided from the same material as the rockbulk material 1 but with a smaller mean grain diameter. The grain sizein the rock bulk material 1 preferably varies in the order of magnitudebetween 30 and 80 mm and the fine-grained bulk material between 15 mmand 30 mm. The air supply channels 8 and 8 a are laid on thefine-grained bulk material 7 so that the heating air is supplied throughan inlet pipe 9 to the main air supply channel 8 and from there isdistributed to the laterally laid air distribution channels 8 a. The airsupply channels 8 and 8 a are preferably manufactured from chamottebrick 10 or from another stone, in particular brick which is suitablefor high temperature. The type of structure of the air channels 8 and 8a is illustrated in FIG. 1B, FIG. 10 and FIG. 1D. FIG. 1F shows detail“A” to illustrate FIG. 1.

Heated air leaves the base channels 8 a through openings 11 (FIG. 1B)and is distributed uniformly over the base of the rock bulk material 1.The hot air flow passes the rock bulk material 1 and releases its heatto said material. Exhaust air channels 12, 12 a located above the rockbulk material 1 have the same structural form as the air supply channels8 and 8 a. The exhaust air channels 12, 12 a are positioned at or on theupper end of the rock bulk material 1. The intermediate spaces betweenthe exhaust air channels 12, 12 a are, as shown and in this respectpreferred, filled with rock bulk material 1. Another fine-grained bulkmaterial 7 a of the same material as the rock bulk material 1 is appliedover the rock bulk material 1 and the exhaust air channels 12, 12 a. Thefine-grained bulk material 7 a is surfaced in two layers by means ofstones 17 such as bricks. Thus, the rock bulk material 1—as explainedadditionally further below—is surrounded spatially by shaped stones.

The exhaust air channels are designated as air collecting channels. Thecooled air stream leaves the heat storage device through an airdischarge pipe 13 connected to the air collecting channels (FIG. 1). Theair discharge pipe 13 is thermally insulated and the insulation 14 isclad by means of a steel pipe 15 which is embedded together with themasonry wall 2 in an ash fill 16 (FIG. 1). The air inlet pipe 9 ispreferably thermally insulated in the same way. The ash fill 16supplements the ash fill 5 laid in the base region below the two lowerlayers of stones 6.

The masonry wall 2 (FIG. 1) laterally surrounds the rock bulk material 1which serves as the heat storage mass. The two layers of stones 6 suchas bricks and stones 17 such as bricks which can form layers in asimilar manner to the stones 6 enclose the heat storage mass at thebottom or top. The heat storage mass together with the air supply andexhaust air channels is therefore surrounded on all sides by rock, inparticular in the form of a masonry wall and/or shaped stones. Theaforementioned encasement of the ash fills 5 and 16 is located aroundthis masonry wall 2, 6 and 17 (FIG. 1). These are provided in athickness AA where the long-term heat storage device 100, as shown in(FIG. 1) and in this respect preferred, has a cylindrical shape with theratio D=H (diameter=height). The only heat bridge towards the outside isthe cylindrical part of the supporting brick masonry wall 2 a where acertain increased heat loss is possible. In the exemplary embodimentthis heat loss is about 3%. The solution according to FIG. 1E howevershows that the brick masonry wall 2 can be provide without a connectionin the form of the supporting brick masonry wall 2 a to the foundation.Therefore this heat bridge towards the outside is avoidable. The entirebrick masonry wall 2, 6 and 17 then lies on the ash fill 5 (FIG. 1E).The rock bulk material 1 is therefore preferably together with the airsupply and air removal enclosed on all sides, in particular with aninterposed shell of shaped stones, by a powder fill, in particular by anash fill 5, 16.

An outer cladding (18) can (according to FIG. 2) be arranged around theinsulating ash layer (16) so that the insulating layer is held and theingress of water and moist air is largely prevented. The outer cladding18 of the long-term heat storage device 100 can be designed in variousways, according to FIG. 2 the outer cladding 18 consists of a pluralityof cylindrical segments 19 which are firmly connected to one another bymeans of flange connections 20. The first base segment 21 is, ifdesired, welded to the base plate (steel plate 3), e.g. in order toprevent the ingress of moisture into the ash layer. The outer cladding18 can be connected in an airtight manner and/or terminated by means ofa cover 22 by flange connections. The cover 22 can be provided with aplurality of openings 22 a and 22 b which can be used to fill the heatstorage device with insulating ash (powder fill 16). The steel pipes 15which are used to protect the air lines 9 and 13 can be embedded in theash layer 16 and be welded to the outer jacket and the segments 19 whichare used for cladding. An insulation of ceramic wool 14 can be providedbetween the air pipelines 9 and 13 and protective pipes 15. Anundisturbed expansion of the air pipelines 9 and 13 during a change intemperature can therefore be made possible. One of the advantages withthis design is that there is no possibility for ingress of moisture intothe ash fill which is important for the problem-free long-term functionof the long-term heat storage device. FIG. 2 a shows a horizontalsection through the long-term heat storage device according to FIG. 2for illustration.

According to FIGS. 3 and 4 another embodiment for the cladding 23 of theheat storage device is reproduced, i.e. the outer cladding 23 consistsof sheet metal shells 24 which are fastened to one another around in acircle and over the height by means of screw connections. The sheetmetal shells 24 form a regular polygon. According to FIG. 4 the planview of the outer cladding 23 of the heat storage device is shown forillustration. Shown here is: a cover 26 with webs 27 and with openings28 which serve for filling the heat storage device with insulating ash.

According to FIG. 5 an improved embodiment of the long-term heat storagedevice with an additional insulation is shown. This can consist ofmineral wool which is applied over the outer cladding 23. The insulatinglayer 29 can also be clad by means of metal sheets, in particular bymeans of so-called trapezoidal sheets 30, in order to protect theinsulation layer 29 from moisture. FIG. 5A shows a horizontal sectionthrough the heat storage device in order to illustrate the insulation.

According to FIG. 6 an embodiment is reproduced in which the long-termheat storage device 100 is disposed in the ground 31, as in the soil.This has the difference from FIG. 1 that the outer cladding is executedas a completely closed cylinder 32. The cylinder 32 is preferably madeof steel sheet and is terminated in a water- and airtight manner. Theheated air is supplied from below through pipeline 33 to the long-termheat storage device and the cooled air is extracted from it throughpipeline 34. FIG. 6A shows a horizontal cross-section through thelong-term heat storage device according to FIG. 6. The remainingstructure of the long-term heat storage device 100 (rock bulk material,air supply and exhaust air guidance, encasement with shaped stones—alsoin the form of concreted walls—and encasement with fine-grained bulkmaterial and possibly sheet metal encasement) can be executed as in thepreceding exemplary embodiments.

Storage capacity: Each storage capacity and each geometrical shape hasan optimal layer thickness of the insulation layer (powder fill 16, 5)for which the heat losses during the storage time are minimal (FIGS. 7Ato 7C). The curves show the dependence of the heat losses/insulationthickness for a circular cylinder geometry (H=D) of the heat storagedevice. Assuming 180 days storage time, a minimum is obtained for acertain minimum insulating layer thickness (16, 5). The optimal economicinsulating layer thickness (16, 5) lies in the range of 0.5 m to 2 m andcan be taken into account for all storage capacities.

According to FIG. 7A, the heat losses of a heat storage device accordingto the invention are plotted as a function of the layer thicknesses of arock bulk material of ash. The heat storage device should be able toabsorb sufficient solar energy in order to fully heat an object having a150 m² living area from, e.g. November to the following April. Thestorage time then lasts from April to the end of October and the heatlosses during the storage time are plotted in the diagram of FIG. 7A.From this diagram it can be seen that for a certain layer thickness, inthis case it is about 1.8 m thick, the heat losses during the storagetime are minimal. From this diagram it can be seen that with increasinglayer thickness of the ash layer, the heat losses increase again. Forthe absolute layer thickness of the ash in the exemplary embodiment,absolute (total) heat losses of about 7,300 kWh are obtained. Theselosses account for about 48% of the stored energy. Here it should alsobe mentioned that the geometry of the heat storage device is a cylinderfor which D=H. If the heat storage device on the other hand has theshape of a cube, the minimum for the heat losses (under, otherwise thesame conditions) is about 58% of the stored energy, i.e. higher than fora cylinder shape with the characteristic D=H. In a heat storage devicewhich has the shape of a sphere, the heat losses on the other hand wereonly around 30%. From this it can be concluded that the geometry of thelong-term heat storage device plays an important role in the heatlosses.

According to FIG. 7B, a long-term heat storage device has been takeninto account as a cylinder (D=H) which can store sufficient thermalenergy during the 6 summer months in order to be able to heat 10,000 m²(200 apartments) from November to April. It can be seen from the diagramin FIG. 7B that the minimum for the ash layer has shifted towards alarger layer thickness. It can further be seen that there is no majordifference in regard to the heat losses for a layer thickness between1.8 m and 4 m. This finding is very important for the economic viabilityof the long-term heat storage device. The heat losses during the storagetime for a layer thickness of 1.8 m are around 38,000 kWh and inrelative terms, relative to the stored energy, are about 3.9%. From thisit can be concluded that with the increasing storage capacity therelative heat losses decrease enormously. In a comparably designedlong-term heat storage device but with a cubic shape, the relative heatlosses are about 4.9%.

In a long-term heat storage device taken as the basis in FIG. 7C for aliving area of 100,000 m² (2000 apartments) and in cylindrical shape(D=H), it can be seen that the minimum for the heat losses is furthershifted towards greater layer thicknesses of the ash. The difference inthe heat losses for 1.8 m layer thickness and 4 m layer thickness iscomparatively small. The heat losses during the storage time are about160,000 kWh as absolute value (1.8 m layer thickness of the ash) and therelative heat losses are about 1.6%. For a cube shape the heat losseswere 180,000 kWh (1.8 m layer thickness of the ash) and the relativeheat losses are about 1.8% whereas for a spherical shape the heat lossesare 93000 kWh (1.8 m layer thickness of the ash) or the relative heatlosses are 0.9%.

It has accordingly been shown that the advantage is clearly in favour ofspherical long-term heat storage devices. After this comes the heatstorage device having a cylindrical shape (D=H). The heat storage devicehaving the cubic shape has the highest heat losses. From the productiontechnology point of view, the spherical heat storage device would becost-intensive and complicated to implement in practice. A cylindricalheat storage device with D=H can be seen as an optimum compromisesolution having an insulating ash layer thickness which lies in theorder of magnitude of 1.4 m to 1.8 m.

Further Exemplary Embodiments

1) The long-term heat storage device according to this invention canachieve a breakthrough in the area of utilization of alternative energybecause an appreciable fraction of the energy problems can be solved bythe long-term storage of solar energy. For example, the whole of Europeas far as Southern England can use the new heat storage system accordingto this invention to solve the energy problems. There is no longer anyneed to attempt to achieve the “DESERTEC” project (Sahara project) inorder to secure power production for Europe. In every Southern Europeancountry (Spain, Italy, Greece, Turkey) it will be possible to buildeconomic solar power plants with continuous power production throughoutthe entire year (day and night) by using the long-term heat storageaccording to this invention. In addition to obtaining power from solarenergy, it is also possible to build solar heating power plants wherethe complete heating requirement can be met by means of solar energy.

2) When using solar energy for heating buildings, the storage capacityof the long-term heat storage device is greater than the storagecapacity for power production because the temperature difference for theheating is (800° C.-100° C.) 700° C. In power production the temperaturedifference in the long-term heat storage device during the decoupling ofheat is at best 500° C. During the decoupling of heat from the long-termheat storage device for heating (in heating power plants), thetemperature difference is 700° C. (max. 800° C.−min. 100° C.). Thisgives a storage capacity of, for example, 450 kWh/m³ whilst in powerproduction with a temperature difference of 500° C. the specific storagecapacity is about 324 kWh/m³.

3) If the long-term heat storage device is designed to heat, forexample, a city having 50,000 dwellings each having 50 m² living areaper dwelling during the winter, and assuming that about 11000 kWh isrequired to heat a dwelling having a living area of 50 m² from Novemberto April of the next year, the dimension of the storage medium can bedetermined as: D=113 m and H=113 m. With an insulating ash layer theoutside diameter of the long-term heat storage device is; D=116 m Ha=116m.

4) Assuming that the sun also shines in winter, according to statisticse.g. for Balkan countries, the energy production can be determinedduring the winter months. The hours of sunshine in the winter months areas follows: in November there are 118 h, in December 54 h, in January 78h, in February 90 h, in March 150 h and in April 208 h. If a goodconcentrator for concentrating the solar energy in a long-term heatstorage device is considered which uses the solar energy in winter, itwould be appropriate to design the heat storage device for 90 daysstorage time, and according to this the dimensions: D=90 m, H=90 m wouldbe obtained for the storage mass. With the insulating ash layer inaddition the outside diameter of the heat storage device becomes: D=93m, H=93 m. In this case the heat losses during storage of the solarenergy (180 days) are 0.6% relative to the total stored energy. It canbe seen from this that the size of the long-term heat storage deviceshould agree with the size and with the number of concentrators in orderto be able to achieve the optimum ratios during usage of the solarenergy. It is quite clear that the new heat storage system forms thebreakthrough in the use of alternative energy.

5) Apart from solar energy it is also possible to store other types ofenergy. For example, heat from biomass, e.g. in the manner that insummer various waste is burnt and the stored energy is used in winterfor heating. Wind energy can also be stored and during windless dayspower can be generated from the stored energy. In this way continuitywould be secured with wind energy. In this case the power need notnecessarily be generated by means of water vapour (the efficiency is toolow) but by means of an air machine which operates with hot air. Thus ahigh efficiency (of the order of magnitude of 40% to 50%) can beachieved accordingly. This air machine must operate according to theCarnot process. The use of a Stirling motor can also be appropriate.

6) Numerically the storage capacity can be determined as follows bymeans of an example: a bulk density of ρs=2100 kg/m³ is desired, thisbulk density corresponds to a void volume of =0.276. From this itfollows that the rock bulk material should preferably be designed as apolydisperse bulk material in order to obtain the required bulk density.The grain size composition is determined in the laboratory. Assumingthat the maximum temperature of the bulk material should be 800° C. andif the mean specific thermal capacity of the rock bulk material between20° C. and 800° C. is 1200 J/kgK, a specific heat storage capacity ofthe rock bulk material in the temperature range between 100° C. and 800°C. of 488 kWh/m³ is obtained. The storage capacity depends on thetemperature level to which the temperature of the bulk material shouldbe able to be decreased due to the heat decoupling. If the focus is onpower production in which water vapour and 20 bar pressure is used (213°C. saturated vapour temperature), it is expedient to cool the heatstorage mass down to 300° C. For this temperature of the heat storagemass a storage capacity of 348 kWh/m³ is obtained.

7) In order to increase the specific storage capacity of the long-termheat storage device, a polydisperse bulk material-for example a granularmaterial comprising at least two grain sizes and/or grain sizedistributions/band widths is used: it will be shown how the specificheat storage capacity of a heat storage device can be influenced by thedesign of the grain spectrum of the bulk material:

Let the true density of the diabase rock be ρw:=2900 kg/m³ and theporosity of the granular bulk material ε:=0.4. The bulk density can thenbe calculated as: ρs:=ρw(1−ε)=1.74×10³ kg/m³. Assuming that the rockbulk material is heated to a maximum temperature tsmax:=800° C. and iscooled to a minimum temperature tmin:=100° C., Δ:=tsmax−tmin=700° C. isobtained. With a specific heat capacity of the rock of C:=1050 J/kgK,the storage capacity of the rock bulk material per unit volume can becalculated as follows:

${\Delta \; Q\text{?}\frac{\left( {\rho \; {s \cdot C \cdot \Delta}\; t} \right)}{3612000}} = {354.07\mspace{14mu} {{kWh}/{m^{3}.\text{?}}}\text{indicates text missing or illegible when filed}}$

The specific storage capacity of the bulk material for ε1:=0.35 andρs1:=ρw(1−ε1)=1.885×10³ kg/m³ is Δ“Q1” “=[”((ρs1−C·Δt))/3612000]=383.576 kWh/m³ and for ε2:=0.3 andρs2:=ρw(1−ε2)=2.03×10³ kg/m³: Δ“Q2” “=[” ((ρs2−C·Δt))/3612000]=413.081kWh/m³ and for ε3:=0.2 and ρs3:=ρw(1−ε3)=2.32×10³ kg/m³: Δ“Q3” “=[”((ρs3−·C·Δt))/3612000]=472.093 kWh/m³. It can be seen from this that thespecific storage capacity of the bulk material increases as a result ofthe increase in the bulk density. At the same time, the increase in thebulk density can be achieved by the skilful mixing of various grainfractions as is shown by means of a numerical example:

We assume that the initial bulk material is composed of the followinggrain fraction: v1:=0.3 m³/m³ volume fraction, d1:=0.03 m grain diameterand ε1:=0.4 void volume, v2:=0.3 m³/m³ volume fraction, d2:=0.05 m graindiameter and L_(E2):=0.4 void volume, v3:=0.4 m³/m³ volume fractiond3:=0.08 m grain diameter and ε3:=0.4 void volume. This should be theinitial bulk material I and the mean grain diameter and the meanspecific surface are can be calculated as dpm:=48 mm and fspm:=75.6m²/m³, εm:=0.4 is the mean void volume and ρs:=ρw(1−ε)=1.74×10³ kg/m³ isthe bulk density of the initial bulk material I.

If a fraction of smaller diameter is added to the initial bulk material,it is found that the bulk density can be increased. Properties of thefine-grained bulk material mixed with the initial bulk material are:

v4:=0.3 m³/m³ volume fraction (30%), d4:=0.005 m grain diameter andε4:=0.4 void volume, v3:=0.4 m³/m³ volume fraction (40%), d5:=0.01 mgrain diameter and ε5:=0.4 void volume, v6:=0.3 m³/m³ volume fraction(30%), d6:=0.02 m grain diameter and ε6:=0.4 void volume (in each casewith single-grain bulk material).

For this fine-grained mixture the mean grain diameter, mean specificsurface area, void volume and bulk density can be calculated:dpml:=8.696 mm, fspmis:=414 m²/m³ emis:=0.4 kg/m³,ρsmis:=ρw(1−εmis)=1.74×10³.

Taking as the basis the heat storage mass having the dimensions:diameter (Dsp) of the heat storage mass Dsp:=4.46 m, H:=4.46 m, theheight of the heat storage mass, volume (Vspm) of the storage massVspm:=69.678 m³, the void volume (VL) in the storage device is then:VL:=εm. Vspm=27.871 m³. If it is further assumed that 50% of the voidvolume is filled with a fine-grained mixture, after mixing the twocoarse and fine-grained bulk materials, this gives: a mean void volumeof the bulk material εtot:=0.28, a mean bulk density of the bulkmaterial ρstot:=ρw(1−εtot)=2.088×10³, a mean specific surface area ofthe bulk material of fsptot:=165.6 m²/m³ and a mean grain diameter ofthe bulk material of dmtot:=26 m. If the new parameters are included inthe calculation, the specific capacity can be calculated as follows:Δ“Qtot” “=[” ((ρstot−C·Δt)/361200]=424.804 kWh/m³. It can be seen fromthis that by adding fine-grained bulk material the increase in thespecific storage capacity is thus: ΔQtot−ΔQ=70.814 kWh/m³, which means a20% increase.

For a grain size spectrum of the mixed bulk material in the sense ofthis invention, the minimal intermediate spacing can be determined forthe grains of minimal diameter and the intermediate spacing of thegrains for the maximum diameter, e.g. for a square and for a rhombicpacking. Herein the rhombic packing is taken into account with ⅔ and thesquare packing with ⅓ volume fraction. This can be determined bycalculation, where the following grain distribution of the fine-grainedgranules is obtained: ⅓ grain spectrum from 12 to 30 mm and ⅔ grainspectrum from 4 mm to 12 mm.

TABLE 1 8) Λeff k = (Λeff/ Λp (W/mK) (W/mK) Δ) W/m²K Thermal t ° C.Effective thermal Heat transfer Dp (mm) conductivity Temperatureconductivity Δ(m) coefficient Particle of the of the bulk of the Bulkdensity through the No. diameter particles material insulating layer ofthe insulation insulating layer 1 0.0011(μ) 0.1 100 0.033 2 0.0165 20.0110(μ) 0.1 100 0.058 2 0.029 3 0.1100(μ) 0.1 100 0.064 2 0.032 4 10.1 100 0.067 2 0.0335 5 5 0.1 100 0.079 2 0.0395 6 10 0.1 100 0.089 20.0445 7 20 0.1 100 0.105 2 0.0525 8 50 0.1 100 0.138 2 0.069

Table 1 gives the dependence of the effective thermal conductivity(Λeff(W/mK)) and corresponding to this the heat transfer coefficientK=(Λeff/Δ) of the insulating bulk material (in this case this is themicronized ash) as a function of the grain diameter where thetemperature in the bulk material and the thermal conductivity of thegrain remain as constant quantities.

TABLE 2 Λeff k = (Λeff/ Λp (W/mK) (W/mK) Δ) W/m²K Thermal t ° C.Effective thermal Heat transfer Dp (mm) conductivity Temperatureconductivity Δ(m) coefficient Particle of the of the bulk of the Bulkdensity through the No. diameter particles material insulating layer ofthe insulation insulating layer 1 0.0011(μ) 1.5 100 0.045 2 0.0225 20.0110(μ) 1.5 100 0.148 2 0.074 3 0.1100(μ) 1.5 100 0.22 2 0.11 4 1 1.5100 0.241 2 0.1205 5 5 1.5 100 0.272 2 0.136 6 10 1.5 100 0.307 2 0.15357 20 1.5 100 0.375 2 0.1875 8 50 1.5 100 0.549 2 0.2745

In Table 2 all the quantities apart from the thermal conductivity of theparticles Λp (W/mK) have remained the same. It can be seen from Table 2that the thermal conductivity of the particles plays a major role in theheat conduction through the bulk material. The value for Λp=1.5 W/mK isdeliberately taken into account to check whether the sand in the desertcan be used as insulating layer because usually no ash is present in thedesert. It can be seen from Table 2 that the thermal conductivity of theinsulating bulk material increases sharply. From this it can beconcluded that as an alternative to micronized ash for the insulatinglayer, a mineral substance having a spongy structure can be used. Bygrinding, the spongy material can be brought into micronized form andused as insulating material. The micronized ash is already present insponge form because the carbon has reacted with the oxygen from thelattice structure as a result of the combustion reaction.

In summary it can therefore be said that apart from micronized ash, amineral substance which has a spongy structure can be used as insulatingmaterial for the heat storage device. Possibly insulating ceramic orwaste from brick can be used which has previously been brought intomicronized form. It further follows from Table 2 that desert sand canpossibly be used as insulating material if the grain is comminuted toless than 10μ. Here the question of the economic viability arises.

In this sense a new possibility is afforded, namely to develop a newtechnology which produces extra micronized ash (100%) from ash-richcoal. In this case, the thermal energy can be used for power productionas usual. This would be a meaningful possibility.

REFERENCE LIST

-   1 Rock bulk material-   2 Masonry wall-   2 a Supporting masonry wall-   3 Plate-   4 Foundation-   5 Powder fill-   6 Stones-   7 Fine-grained bulk material-   7 a Fine-grained bulk material-   8 Air supply channel-   8 a Air supply distribution channels-   9 Inlet pipe for hot air-   10 Chamotte brick-   11 Outlet openings for hot air-   12 Exhaust air channel-   13 Air discharge pipe-   14 Insulation of air lines-   15 Steel pipe-   16 Powder fill-   17 Stones-   18 Outer cladding-   19 Cladding segment-   20 Flange connection-   21 Base segment of outer cladding-   22 Cover for the outer cladding-   22 a Small opening-   22 b Large opening-   23 Outer cladding-   24 Sheet metal shell-   25 Polygon of sheet metal shells (24)-   26 Cover-   27 Web-   28 Openings-   29 Insulation-   30 Trapezoidal sheet metal-   31 Ground-   32 Cylinder-   33 Air pipeline hot air inlet-   34 Air pipeline air outlet-   100 Long-term heat storage device-   Delta A Thickness-   D Diameter-   H Height

1. Long-term heat storage device for the long-term storage of solarenergy and other types of energy comprising a heat storage mass,characterized by a thermally insulated solid bed of a rock bulk materialserving as heat storage mass, comprising a material suitable for highoperating temperatures, in particular for temperatures between 100° C.and 1000° C., and having a high specific heat capacity greater than 600J/kgK, a high thermal conductivity higher than 1 W/mK and a high densitygreater than 2 kg/dm³, the rock bul material being of volcanic originsuch as diabase, basalt, granite and/or gneiss.
 2. The long-term heatstorage device according to claim 1, characterized by a thermallyinsulated solid bed of a rock bulk material serving as heat storagemass, the rock bulk material forms a polydisperse bulk material wherebythe void volume of the rock bulk material (granular material) having afirst grain size or grain size distribution takes up a granular materialhaving a second grain size or grain size distribution.
 3. The long-termheat storage device according to claim 1, characterized by a thermallyinsulated solid bed of a rock bulk material serving as heat storagemass, the rock bulk material together with an air supply and air removalis partially or fully surrounded with an interposed shell of shapedstones, by a bulk powder fill such as an ash fill.
 4. The long-term heatstorage device according to claim 1, characterized by a thermallyinsulated solid bed of a rock bulk material serving as heat storagemass, the rock bulk material together with an air supply and air removalis partially or fully surrounded by an in particular cylindrical shellof shaped stones such as by a masonry wall.
 5. The long-term heatstorage device according to claim 1, characterized in that the rock bulkmaterial lays directly or indirectly on a bulk material comprising inparticular micronized powder of a solid having a porous or fibrousstructure, in particular of a group comprising micronized ash, powderhaving a large interior porous structure, sand having a porousstructure, ground clay, ground brick, ground reacted solids such asactivated charcoal or ground rock having a porous (spongy) structure aswell as various organic substances having a fibrous structure.
 6. Thelong-term heat storage device according to claim 1, characterized inthat one or more layers of stone such as bricks, is/are disposed on thebulk material.
 7. The long-term heat storage device according to claim1, characterized in that a fine-grained bulk material is disposedunderneath the rock bulk material and which has the same origin as therock bulk material.
 8. The long-term heat storage device according toclaim 1, characterized in that the rock bulk material such as a masonrywall surrounding the rock bulk material is surrounded by a bulk powderfill such as micronized ash.
 9. The long-term heat storage deviceaccording to claim 1, characterized in that incoming air distributionchannels are disposed below the rock bulk material such as on afine-grained bulk material.
 10. The long-term heat storage deviceaccording to claim 9, characterized in that a main air channel and airdistribution channels are shaped so that outlet openings for hot air areprovided at the air distribution channels so that the air is distributeduniformly over the entire base of the rock bulk material.
 11. Thelong-term heat storage device according to claim 1, characterized inthat at the upper end of the rock bulk material exhaust air channels arelaid so that the outflowing air is collected over the entirecross-section of the rock bulk material in the channels and is removedoutwards.
 12. The long-term heat storage device according to claim 10,characterized in that the intermediate spaces of the air supply channelsand/or the air collecting channels are filled with rock bulk material.13. The long-term heat storage device according to claim 11,characterized in that the space above the air collecting channels isfilled with fine-grained bulk material.
 14. The long-term heat storagedevice according to claim 13, characterized in that the fine-grainedbulk material is constructed in two layers by means of the brick. 15.The long-term heat storage device according to claim 14, characterizedin that a bulk powder fill is provided as insulation over the rock bulkmaterial such as over the stones.
 16. The long-term heat storage deviceaccording to claim 15, characterized in that the bulk material has thesame thickness as the bulk material which is disposed laterally aroundthe brick masonry wall.
 17. The long-term heat storage device accordingto claim 9, characterized in that an insulated air supply pipelineinserted in a protective pipe is provided for the air inlet into the airsupply distribution channels and/or for the air outlet from the exhaustair distribution channels which pipeline is provided with a cladding.18. The long-term heat storage device according to claim 1,characterized in that a cladding made of sheet metal shells is providedaround the insulating ash layer surrounding the rock bulk material. 19.The long-term heat storage device according to claim 18, characterizedin that a covering of the rock bulk material of the masonry wall, thestones and the insulating ash layer is provided by means of the cover insuch a manner that the cover is connected to the outer cladding in awatertight manner.
 20. The long-term heat storage device according toclaim 19, characterized in that filling openings are provided on thecover, in order to fill the space between the outer cladding and themasonry wall with bulk powder fill.
 21. The long-term heat storagedevice according to claim 20, characterized in that the entire masonrywall and the rock bulk material lies on a bulk powder fill.
 22. Thelong-term heat storage device according to claim 11, characterized inthat the thickness of the bulk powder fill is selected according to thestorage capacity and according to the geometry.
 23. The long-term heatstorage device according to claim 1, characterized in that the long-termheat storage device has the shape of a cylinder in which the diameterand the height of the heat storage mass are equal (D=H) and afterinsulation of the heat storage mass with bulk powder fill the diameterand the height of the cylinder are approximately the same (D=H).
 24. Thelong-term heat storage device according to claim 1, characterized inthat the heat storage device lies on a plate to prevent ingress ofmoisture into the insulation or into the heat storage mass.
 25. Thelong-term heat storage device according to claim 1, characterized inthat the heat storage device is laid underground so that the heatstorage device is enclosed in a sleeve which is watertight in order toprevent the penetration of moisture into the insulation or into the heatstorage mass.
 26. The long-term heat storage device according to claim1, characterized in that the rock bulk material has a bulk density of atleast 1600 kg/m³.
 27. Method for the long-term heat storage of solarenergy and other types of energy with changing availability by means ofa long-term heat storage device in which the solar energy or the othertype of energy with changing availability is initially transferred to aheat transfer medium and then the thermal energy is transported into thelong-term heat storage device and to the heat storage mass,characterized in that a granular material (rock bulk material) has ahigh specific heat capacity, high thermal conductivity and/or highdensity, and has a volcanic origin such as basalt rock, diabase rock,granite rock and/or gneiss rock, and is provided in a solid bed as heatstorage mass, that the granular material is insulated in a suitablemanner so that the heat losses remain low during a long period of time,wherein the heat insulating material has a plurality of contactresistances and/or its thermal conductivity is low and/or its structureis spongy, fibrous or porous and comprises a micronized powder such asmicronized ash and that the granular material is heated to a hightemperature by means of a heat transfer medium interacting with thegranular material by means of air.